The laboratory rat is a popular experimental animal to study physiological and pathological interventions on cardiac function, but seldom are these studied in both sexes simultaneously. Few data are available comparing cardiac function in male and female mammals, but preliminary data from our laboratory would suggest basic differences in the function of male and female hearts. The proposed project would study cardiac function under both baseline and stress conditions to uncover degrees of cardiac reserve in male and female hearts. Studies will be performed in the isolated working heart to examine ventricular function, pressure-volume relationships and myocardial metabolism under highly controlled conditions of preload, afterload and heart rate. Functional reserve will be determined under conditions of increased preload and heart rate. Comparisons will be made in male and female rats of the same age where heart size is different and in other models where heart size is the same. Other studies in gonadectomized animals with and without replacement of estrogen or testosterone will be conducted to examine the role of the sex hormones on the ultimate mechanical properties of the adult myocardium. If our studies uncover basic differences in male and female hearts then further studies are planned to examine possible biochemical mechanisms that may be responsible. These studies will provide basic information regarding ventricular function in male and female rats which should be of value in the interpretation of experimental findings where the rat heart is used as a model.